42. Markers

A marker is a Lisp object used to specify a position in a buffer
relative to the surrounding text. A marker changes its offset from the
beginning of the buffer automatically whenever text is inserted or
deleted, so that it stays with the two characters on either side of it.

42.1 Overview of Markers

A marker specifies a buffer and a position in that buffer. The marker
can be used to represent a position in the functions that require one,
just as an integer could be used. See section 41. Positions, for a complete
description of positions.

A marker has two attributes: the marker position, and the marker
buffer. The marker position is an integer that is equivalent (at a
given time) to the marker as a position in that buffer. But the
marker's position value can change often during the life of the marker.
Insertion and deletion of text in the buffer relocate the marker. The
idea is that a marker positioned between two characters remains between
those two characters despite insertion and deletion elsewhere in the
buffer. Relocation changes the integer equivalent of the marker.

Deleting text around a marker's position leaves the marker between the
characters immediately before and after the deleted text. Inserting
text at the position of a marker normally leaves the marker in front of
the new text--unless it is inserted with insert-before-markers
(see section 43.4 Inserting Text).

Insertion and deletion in a buffer must check all the markers and
relocate them if necessary. This slows processing in a buffer with a
large number of markers. For this reason, it is a good idea to make a
marker point nowhere if you are sure you don't need it any more.
Unreferenced markers are garbage collected eventually, but until then
will continue to use time if they do point somewhere.

Because it is common to perform arithmetic operations on a marker
position, most of the arithmetic operations (including + and
-) accept markers as arguments. In such cases, the marker
stands for its current position.

Note that you can use extents to achieve the same functionality, and
more, as markers. (Markers were defined before extents, which is why
they both continue to exist.) A zero-length extent with the
detachable property removed is almost identical to a marker.
(See section 47.3 Extent Endpoints, for more information on zero-length extents.)

In particular:

In order to get marker-like behavior in a zero-length extent, the
detachable property must be removed (otherwise, the extent
will disappear when text near it is deleted) and exactly one
endpoint must be closed (if both endpoints are closed, the extent
will expand to contain text inserted where it is located).

If a zero-length extent has the end-open property but not
the start-open property (this is the default), text inserted
at the extent's location causes the extent to move forward, just
like a marker.

If a zero-length extent has the start-open property but not
the end-open property, text inserted at the extent's location
causes the extent to remain before the text, like what happens to
markers when insert-before-markers is used.

Markers end up after or before inserted text depending on whether
insert or insert-before-markers was called. These
functions do not affect zero-length extents differently; instead,
the presence or absence of the start-open and end-open
extent properties determines this, as just described.

Markers are automatically removed from a buffer when they are no
longer in use. Extents remain around until explicitly removed
from a buffer.

Many functions are provided for listing the extents in a buffer or
in a region of a buffer. No such functions exist for markers.

Here are examples of creating markers, setting markers, and moving point
to markers:

;; Make a new marker that initially does not point anywhere:
(setq m1 (make-marker))
=> #<marker in no buffer>
;; Set m1 to point between the 99th and 100th characters
;; in the current buffer:
(set-marker m1 100)
=> #<marker at 100 in markers.texi>
;; Now insert one character at the beginning of the buffer:
(goto-char (point-min))
=> 1
(insert "Q")
=> nil
;; m1 is updated appropriately.
m1
=> #<marker at 101 in markers.texi>
;; Two markers that point to the same position
;; are not eq, but they are equal.
(setq m2 (copy-marker m1))
=> #<marker at 101 in markers.texi>
(eq m1 m2)
=> nil
(equal m1 m2)
=> t
;; When you are finished using a marker, make it point nowhere.
(set-marker m1 nil)
=> #<marker in no buffer>

42.2 Predicates on Markers

You can test an object to see whether it is a marker, or whether it is
either an integer or a marker or either an integer, a character, or a
marker. The latter tests are useful in connection with the arithmetic
functions that work with any of markers, integers, or characters.

Function:markerpobject

This function returns t if object is a marker, nil
otherwise. Note that integers are not markers, even though many
functions will accept either a marker or an integer.

Function:integer-or-marker-pobject

This function returns t if object is an integer or a marker,
nil otherwise.

Function:integer-char-or-marker-pobject

This function returns t if object is an integer, a
character, or a marker, nil otherwise.

Function:number-or-marker-pobject

This function returns t if object is a number (either kind)
or a marker, nil otherwise.

Function:number-char-or-marker-pobject

This function returns t if object is a number (either
kind), a character, or a marker, nil otherwise.

42.3 Functions That Create Markers

When you create a new marker, you can make it point nowhere, or point
to the present position of point, or to the beginning or end of the
accessible portion of the buffer, or to the same place as another given
marker.

Function:make-marker

This functions returns a newly created marker that does not point
anywhere.

(make-marker)
=> #<marker in no buffer>

Function:point-marker&optional dont-copy-p buffer

This function returns a marker that points to the present position of
point in buffer, which defaults to the current buffer.
See section 41.1 Point. For an example, see copy-marker, below.

Internally, a marker corresponding to point is always maintained.
Normally the marker returned by point-marker is a copy; you
may modify it with reckless abandon. However, if optional argument
dont-copy-p is non-nil, then the real point-marker is
returned; modifying the position of this marker will move point.
It is illegal to change the buffer of it, or make it point nowhere.

Function:point-min-marker&optional buffer

This function returns a new marker that points to the beginning of the
accessible portion of buffer, which defaults to the current
buffer. This will be the beginning of the buffer unless narrowing is in
effect. See section 41.4 Narrowing.

Function:point-max-marker&optional buffer

This function returns a new marker that points to the end of the
accessible portion of buffer, which defaults to the current
buffer. This will be the end of the buffer unless narrowing is in
effect. See section 41.4 Narrowing.

Here are examples of this function and point-min-marker, shown in
a buffer containing a version of the source file for the text of this
chapter.

If passed a marker as its argument, copy-marker returns a
new marker that points to the same place and the same buffer as does
marker-or-integer. If passed an integer as its argument,
copy-marker returns a new marker that points to position
marker-or-integer in the current buffer.

If passed an integer argument less than 1, copy-marker returns a
new marker that points to the beginning of the current buffer. If
passed an integer argument greater than the length of the buffer,
copy-marker returns a new marker that points to the end of the
buffer.

An error is signaled if marker-or-integer is neither a marker nor
an integer.

Optional second argument marker-type specifies the insertion type
of the new marker; see marker-insertion-type.

42.5 Changing Marker Positions

This section describes how to change the position of an existing
marker. When you do this, be sure you know whether the marker is used
outside of your program, and, if so, what effects will result from
moving it--otherwise, confusing things may happen in other parts of
Emacs.

Function:set-markermarker position &optional buffer

This function moves marker to position
in buffer. If buffer is not provided, it defaults to
the current buffer.

position can be a marker, an integer or nil. If
position is an integer, set-marker moves marker to
point before the positionth character in buffer. If
position is nil, marker is made to point nowhere.
Then it no longer slows down editing in any buffer. If position
is less than 1, marker is moved to the beginning of buffer.
If position is greater than the size of buffer, marker
is moved to the end of buffer.

42.6 The Mark

One special marker in each buffer is designated the mark. It
records a position for the user for the sake of commands such as
C-w and C-x TAB. Lisp programs should set the mark
only to values that have a potential use to the user, and never for
their own internal purposes. For example, the replace-regexp
command sets the mark to the value of point before doing any
replacements, because this enables the user to move back there
conveniently after the replace is finished.

Once the mark "exists" in a buffer, it normally never ceases to
exist. However, it may become inactive, and usually does so
after each command (other than simple motion commands and some
commands that explicitly activate the mark). When the mark is active,
the region between point and the mark is called the active region
and is highlighted specially.

Many commands are designed so that when called interactively they
operate on the text between point and the mark. Such commands work
only when an active region exists, i.e. when the mark is active.
(The reason for this is to prevent you from accidentally deleting
or changing large chunks of your text.) If you are writing such
a command, don't examine the mark directly; instead, use
interactive with the `r' specification. This provides the
values of point and the mark as arguments to the command in an
interactive call, but permits other Lisp programs to specify arguments
explicitly, and automatically signals an error if the command is called
interactively when no active region exists. See section 25.2.2 Code Characters for interactive.

Each buffer has its own value of the mark that is independent of the
value of the mark in other buffers. (When a buffer is created, the mark
exists but does not point anywhere. We consider this state as "the
absence of a mark in that buffer.") However, only one active region can
exist at a time. Activating the mark in one buffer automatically
deactivates an active mark in any other buffer. Note that the user can
explicitly activate a mark at any time by using the command
activate-region (normally bound to M-C-z) or by using the
command exchange-point-and-mark (normally bound to C-x C-x),
which has the side effect of activating the mark.

Some people do not like active regions, so they disable this behavior
by setting the variable zmacs-regions to nil. This makes
the mark always active (except when a buffer is just created and the
mark points nowhere), and turns off the highlighting of the region
between point and the mark. Commands that explicitly retrieve the value
of the mark should make sure that they behave correctly and consistently
irrespective of the setting of zmacs-regions; some primitives are
provided to ensure this behavior.

In addition to the mark, each buffer has a mark ring which is a
list of markers containing previous values of the mark. When editing
commands change the mark, they should normally save the old value of the
mark on the mark ring. The variable mark-ring-max specifies the
maximum number of entries in the mark ring; once the list becomes this
long, adding a new element deletes the last element.

Function:mark&optional force buffer

This function returns buffer's mark position as an integer.
buffer defaults to the current buffer if omitted.

If the mark is inactive, mark normally returns nil.
However, if force is non-nil, then mark returns the
mark position anyway--or nil, if the mark is not yet set for
the buffer.

(Remember that if zmacs-regions is nil, the mark is
always active as long as it exists, and the force argument
will have no effect.)

If you are using this in an editing command, you are most likely making
a mistake; see the documentation of set-mark below.

Function:mark-marker&optional force buffer

This function returns buffer's mark. buffer defaults to the
current buffer if omitted. This is the very marker that records the
mark location inside XEmacs, not a copy. Therefore, changing this
marker's position will directly affect the position of the mark. Don't
do it unless that is the effect you want.

If the mark is inactive, mark-marker normally returns nil.
However, if force is non-nil, then mark-marker
returns the mark anyway.

Like any marker, this marker can be set to point at any buffer you like.
We don't recommend that you make it point at any buffer other than the
one of which it is the mark. If you do, it will yield perfectly
consistent, but rather odd, results.

Function:set-markposition &optional buffer

This function sets buffer's mark to position, and activates
the mark. buffer defaults to the current buffer if omitted. The
old value of the mark is not pushed onto the mark ring.

Please note: Use this function only if you want the user to
see that the mark has moved, and you want the previous mark position to
be lost. Normally, when a new mark is set, the old one should go on the
mark-ring. For this reason, most applications should use
push-mark and pop-mark, not set-mark.

Novice XEmacs Lisp programmers often try to use the mark for the wrong
purposes. The mark saves a location for the user's convenience. An
editing command should not alter the mark unless altering the mark is
part of the user-level functionality of the command. (And, in that
case, this effect should be documented.) To remember a location for
internal use in the Lisp program, store it in a Lisp variable. For
example:

This function exchanges the positions of point and the mark.
It is intended for interactive use. The mark is also activated
unless dont-activate-region is non-nil.

Function:push-mark&optional position nomsg activate buffer

This function sets buffer's mark to position, and pushes a
copy of the previous mark onto mark-ring. buffer defaults
to the current buffer if omitted. If position is nil, then
the value of point is used. push-mark returns nil.

If the last global mark pushed was not in buffer, also push
position on the global mark ring (see below).

The function push-mark normally does not activate the
mark. To do that, specify t for the argument activate.

A `Mark set' message is displayed unless nomsg is
non-nil.

Function:pop-mark

This function pops off the top element of mark-ring and makes
that mark become the buffer's actual mark. This does not move point in
the buffer, and it does nothing if mark-ring is empty. It
deactivates the mark.

The return value is not meaningful.

Variable:mark-ring

The value of this buffer-local variable is the list of saved former
marks of the current buffer, most recent first.

mark-ring
=> (#<marker at 11050 in markers.texi>
#<marker at 10832 in markers.texi>
...)

User Option:mark-ring-max

The value of this variable is the maximum size of mark-ring. If
more marks than this are pushed onto the mark-ring,
push-mark discards an old mark when it adds a new one.

In additional to a per-buffer mark ring, there is a global mark
ring. Marks are pushed onto the global mark ring the first time you
set a mark after switching buffers.

Variable:global-mark-ring

The value of this variable is the list of saved former global marks,
most recent first.

User Option:mark-ring-max

The value of this variable is the maximum size of
global-mark-ring. If more marks than this are pushed onto the
global-mark-ring, push-mark discards an old mark when it
adds a new one.

Command:pop-global-mark

This function pops a mark off the global mark ring and jumps to that
location.

42.7 The Region

The text between point and the mark is known as the region.
Various functions operate on text delimited by point and the mark, but
only those functions specifically related to the region itself are
described here.

When zmacs-regions is non-nil (this is the default), the
concept of an active region exists. The region is active when the
corresponding mark is active. Note that only one active region at a
time can exist--i.e. only one buffer's region is active at a time.
See section 42.6 The Mark, for more information about active regions.

User Option:zmacs-regions

If non-nil (the default), active regions are used. See section 42.6 The Mark,
for a detailed explanation of what this means.

A number of functions are provided for explicitly determining the
bounds of the region and whether it is active. Few programs need to use
these functions, however. A command designed to operate on a region
should normally use interactive with the `r' specification
to find the beginning and end of the region. This lets other Lisp
programs specify the bounds explicitly as arguments and automatically
respects the user's setting for zmacs-regions.
(See section 25.2.2 Code Characters for interactive.)

Function:region-beginning&optional buffer

This function returns the position of the beginning of buffer's
region (as an integer). This is the position of either point or the
mark, whichever is smaller. buffer defaults to the current buffer
if omitted.

If the mark does not point anywhere, an error is signaled. Note that
this function ignores whether the region is active.

Function:region-end&optional buffer

This function returns the position of the end of buffer's region
(as an integer). This is the position of either point or the mark,
whichever is larger. buffer defaults to the current buffer if
omitted.

If the mark does not point anywhere, an error is signaled. Note that
this function ignores whether the region is active.

Function:region-exists-p

This function is non-nil if the region exists. If active regions
are in use (i.e. zmacs-regions is true), this means that the
region is active. Otherwise, this means that the user has pushed a mark
in this buffer at some point in the past. If this function returns nil,
a function that uses the `r' interactive specification will cause
an error when called interactively.

Function:region-active-p

If zmacs-regions is true, this is equivalent to
region-exists-p. Otherwise, this function always returns false.
This function is used by commands such as fill-paragraph-or-region
and capitalize-region-or-word, which operate either on the active
region or on something else (e.g. the word or paragraph at point).

Variable:zmacs-region-stays

If a command sets this variable to true, the currently active region
will remain activated when the command finishes. (Normally the region is
deactivated when each command terminates.) If zmacs-regions is
false, however, this has no effect. Under normal circumstances, you do
not need to set this; use the interactive specification `_'
instead, if you want the region to remain active.

Function:zmacs-activate-region

This function activates the region in the current buffer (this is
equivalent to activating the current buffer's mark). This will normally
also highlight the text in the active region and set
zmacs-region-stays to t. (If zmacs-regions is
false, however, this function has no effect.)

Function:zmacs-deactivate-region

This function deactivates the region in the current buffer (this is
equivalent to deactivating the current buffer's mark). This will
normally also unhighlight the text in the active region and set
zmacs-region-stays to nil. (If zmacs-regions is
false, however, this function has no effect.)

Function:zmacs-update-region

This function updates the active region, if it's currently active. (If
there is no active region, this function does nothing.) This has the
effect of updating the highlighting on the text in the region; but you
should never need to call this except under rather strange
circumstances. The command loop automatically calls it when
appropriate. Calling this function will call the hook
zmacs-update-region-hook, if the region is active.

Variable:zmacs-activate-region-hook

This normal hook is called when a region becomes active. (Usually this
happens as a result of a command that activates the region, such as
set-mark-command, activate-region, or
exchange-point-and-mark.) Note that calling
`zmacs-activate-region' will call this hook, even if the region is
already active. If zmacs-regions is false, however, this hook
will never get called under any circumstances.

Variable:zmacs-deactivate-region-hook

This normal hook is called when an active region becomes inactive.
(Calling `zmacs-deactivate-region' when the region is inactive will
not cause this hook to be called.) If zmacs-regions is
false, this hook will never get called.

Variable:zmacs-update-region-hook

This normal hook is called when an active region is "updated" by
zmacs-update-region. This normally gets called at the end
of each command that sets zmacs-region-stays to t,
indicating that the region should remain activated. The motion
commands do this.